Thanks for all the replies! I guess the easiest way to "double cast" would be to cast once from your wand, and have a 2nd copy of the spell prepared in your hand.
Casting dissolve/explode on wands seems to be an awfully popular move, and it is good to know that you can "unbind" a spell without having to put a new spell there.
[Edit] Oh, how about this idea? Since a wand is a one-handed equipment ("Weapon OR Shield"), could you dual-wield?
Two fireblast wands with a fireshaper ring -> pretty sweet for damage.
Two incantation wands with teleport -> teleport a friendly creature, and then teleport yourself.